1. Field of the Invention
This invention relates to the field of voltage regulators, and particularly to trimmable resistive networks used in the feedback loop of a voltage regulator to establish its output voltage.
2. Description of the Related Art
A conventional series pass voltage regulator is shown in FIG. 1. A supply voltage V.sub.in is connected to the collector 10 of a pass transistor 12, typically a bipolar transistor, and an output voltage V.sub.out is taken at the transistor's emitter 14. The output voltage is regulated by controlling pass transistor 12 via its base terminal 16. Regulation is accomplished with a feedback loop: the output voltage is fed back to the inverting input 18 of an error amplifier 20, usually after being divided down with a voltage divider 22. A voltage reference V.sub.ref is connected to the non-inverting input 24 of the amplifier. In operation, the amplifier's output 26 drives pass transistor 12 as needed to make the voltage at its inverting and non-inverting inputs equal. By dividing down V.sub.out, the voltage divider 22 enables the regulator to produce an output voltage V.sub.out that is greater than the reference voltage V.sub.ref.
It is occasionally desirable to manufacture a voltage regulator which is capable of producing a number of different output voltages without changing any components or component values, with the desired output voltage selected during fabrication with a trimming step. In the regulator of FIG. 1, this capability is provided with the use of trimmable voltage divider 22. Divider 22 is made from four series-connected resistors Ra-Rd, each of which has a respectivey "severable link" La-Ld connected across it; the four resistors are connected between the regulator output voltage V.sub.out and a fixed voltage which is typically ground. The divider produces a feedback voltage V.sub.fb at a divider tap point 28. The links are severable with a laser, with the aforementioned trimming step used to sever the links as necessary to produce a desired output voltage.
To date, the trimmable voltage dividers found in regulator feedback loops have been arranged as shown in FIG. 1--i.e., with trimmable resistances provided on both sides of divider tap 28. This configuration affords several advantages: a number of division ratios 2.sup.n is made possible, with n being the number of links in the divider. Further, because the trimmability is distributed on either side of tap 28, the change in impedance seen by amplifier 20 over the range of attainable division ratios is kept small.
However, the standard trimmable divider configuration shown in FIG. 1 also has disadvantages. Because there are severable links on either side of the divider tap, the effect on output voltage had by severing the links above the tap ("upper links") is dependent on the status of the links below the tap ("lower links"). Severing more of the lower links increases the net resistance below the tap, which decreases the effect on V.sub.out of severing upper links. Also, while output voltage increases as upper links are severed, it decreases as lower links are severed. These various and contradictory effects on output voltage resulting from the placement of links on either side of the divider tap make the determination of the link configuration needed to produce a desired output voltage confusing and difficult.
Another disadvantage inherent in resistive networks of the type shown in FIG. 1 is the limited range of obtainable output voltages. Because links on opposite sides of the tap can reduce a given link's effect, the range of obtainable output voltages as the number of severed links goes from few to many is limited.